The overall objective is to better understand structure-function interactions in the design and capacity of muscles for blood-tissue exchange. We developed methods to assess several aspects of muscle capillary-fiber morphometrics. We will train students to use these methods, together with histochemical characterization of capillary heterogeneity to explore myocardial structural capacity for O2 flux and study its limit in relation to fiber oxygen demand. This project will focus on rat and rabbit heart, with comparison with birds and bats to explore natural differences in muscle aerobic capacity and degree of capillarization. Our central hypothesis is that it is the size of the capillary-fiber interface, not diffusion distances, which determine O2 flux rates in aerobic muscles. Data in the highly aerobic myocardium will be important correlates to our ongoing studies of skeletal muscles with large differences in fiber type distribution, clustering and aerobic capacity, including locomotory muscles of terrestrial mammals and flight muscles in bird and bat. It should provide new insights in the understanding of key aspects of structure-function correlations in muscle capacity for blood-tissue transfer, and should help in the understanding and management of human response to impaired tissue oxygenation.